The present invention relates to planetary gear systems for automatic transmissions, especially of a type having three planetary gear sets.
Japanese patent provisional publication No. 62-83541 discloses a conventional planetary gear system for providing five or more forward speeds. This planetary gear system has a main planetary gear train composed of two planetary gear sets for providing four forward speeds, and a secondary planetary gear train which is composed of one planetary gear set, and which is connected in series with the main planetary gear train.
Japanese patent provisional publication No. 63-47542 discloses a planetary gear system for a transaxle of an F.F. vehicle. This planetary gear system has a main planetary gear train coaxial with an input shaft, and a secondary planetary gear train coaxial with an output shaft which is parallel to the input shaft. The input and output shafts are connected by gearing.
However, the latter system requires two friction devices for the secondary planetary gear train in addition to five friction devices for the main planetary gear train, so that the number of the required friction devices is relatively great.
Furthermore, the gear ratio finally obtained by the gear arrangement in which the main planetary gear train is connected in series with the secondary planetary gear train, is determined by a product between the gear ratio of the main planetary gear train and the gear ratio of the secondary gear train. Therefore, the gear ratios of the conventional gear system tend to be too high or too low, and it is difficult to set the gear ratios of all the forward speeds equal to the optimum values.
The conventional gear systems are so arranged that the main and secondary gear trains are shifted independently. Therefore, these systems require a complicated control unit for controlling the friction devices in order to reduce a shift shock, especially to reduce a shift shock when both planetary gear trains are shifted simultaneously.
Japanese patent provisional publication No. 62-155356 discloses another conventional planetary gear system for F.F. vehicles. This conventional gear system is composed of one planetary gear set and a plurality of pairs of mating gears of a parallel shaft type. However, the parallel shaft type gears of this conventional system increases the size of the transmission. Some of the friction devices must have a sufficiently great torque capacity to endure torque of a considerable magnitude. Furthermore, some clutches are arranged in series with the parallel shaft type gears, so that the size of the system is increased. The controllability of this conventional gear system is relatively poor because some clutches are switched while they are rotating.